1. Field of the Invention
The present invention relates to analog to digital converters, and more particularly, to converters for converting a time varying analog input.
2. Description of the Prior Art
An analog to digital converter is a device which, as its name suggests, converts an analog input signal to an equivalent digital representation. There are several types of analog to digital converters (hereinafter "A/D converter") such as the counter A/D converter and the successive approximation A/D converter.
One of the most popular types is the dual slope A/D converter. Basically, the dual slope converter converts an analog input signal to a digital representation by integrating the input signal over a fixed time period known as the "integrate cycle". During the integrate cycle, the integrated signal rises from an initial value to a second value. The resultant signal is then "deintegrated" during a "deintegrate cycle", by substituting a reference signal of the opposite polarity for the input signal, which causes the integrated signal to return to its initial value. This second time period is also sometimes referred to as the "timing cycle".
As will be more fully explained below, the duration of the timing cycle is proportional to the magnitude of the input signal. The duration of the timing cycle can be measured with a digital counter such that the output of the digital counter at the end of the timing cycle is a digital representation of the analog input signal. Other A/D converters known as "multislope converters" use variations of the scheme outlined above, but the basic principle remains the same.
A constant analog input signal is usually converted into the corresponding digital representation of that input signal. However, time varying input signals such as a sinusoidal AC (alternating current) signal, are typically converted to the average value of the input signal. Often a more useful measurement, particularly for sinusoidal input signals, is the RMS or root mean square of the signal. The root mean square of a function, in general, is defined as the square root of the time average of the square of the function. This may be represented as follows for the function x(t): ##EQU1##
It is known that the RMS value is related to the average value of a sinusoidal signal by a factor of .pi./2.sqroot.2 which is approximately 1.11072. Accordingly, in order to determine the RMS voltage, for example, instead of the average voltage, it has been suggested to multiply the average voltage by this factor of .pi./2.sqroot.2. One technique of accomplishing this is to first buffer the sinusoidal AC input signal and then rectify it to produce a signal having a magnitude which is the average value of the AC signal. This average signal can then be multiplied by the factor of .pi./2.sqroot.2 by means of an amplifier.
However, this is an inherently imprecise method due to the inaccuracies typically associated with an amplifier. For example, the output voltage of an amplifier for a given input voltage often drifts with the passage of time and due to variations in temperature. Futhermore, amplifiers usually have an offset voltage resulting from imperfections in the manufacture of the device. As a result, the amplifier usually cannot be depended upon to multiply the average signal precisely by the desired factor of .pi./2.sqroot.2 all the time. Instead, the multiplication factor often drifts to some other value. Accordingly, this method is not particularly useful for applications requiring a high degree of precision.
Furthermore, these multiplier amplifiers typically require several components for noise reduction purposes and the like, which usually are not, as a practical matter, integrated on a semiconductor chip. Thus, an integrated circuit A/D converter with a multiplier amplifier usually requires many external components which increases the cost and complexity of manufacturing the A/D converter.